[Candidate.class.inc] Allow null Sex for scanner candidates (#7058)

Fix bug where PHP class could not handle null Sex value for scanner candidates.

php/libraries/Candidate.class.inc

@@ -103,7 +103,7 @@ class Candidate implements \LORIS\StudyEntities\AccessibleResource,
         foreach ($row as $key=>$value) {
             switch ($key){
             case 'Sex':
-                $this->candidateInfo[$key] = new Sex($value);
+                $this->candidateInfo[$key] = empty($value) ? null : new Sex($value);
                 break;
             case 'Entity_type':
                 $this->candidateInfo[$key] = new EntityType($value);